1. Field of the Invention
The present invention relates to an improvement on a process for machining an edge portion of a ceramic article preform, particularly, an edge portion of a ceramic article preform having a rotationary symmetrical edge.
2. Related Art Statement
When a workpiece such as a preform of a ceramic article is to be ground, a machining order and machining directions are subjected to arbitrary discretion of a worker, or such machining order and machining directions are selected from the standpoint of shortening the machining time. When the ceramic article preform is to be machined by a machining tool such as a grinding stone in a traverse machining, no chipping occurs at a side of a machining face of the ceramic article preform which the machining tool enters because mainly compression stress applies, whereas because tensile stress applies to a side of the machining face of the preform through which the grinding tool leaves, chipping inevitably occurs there. Since the ceramic article is conventionally finished after being ground, it is an actually common practice to machine the preform in such a machining order and machining directions as causing chipping without taking any countermeasure to preventing the chipping. However, if the ceramic article is chipped, it is considered that the ceramic article, particularly a structural article such as an engine valve, begins to be broken from a chipped portion.
The above chipping phenomenon is explained with reference to the conventional process for grinding a preform of a ceramic valve by way of example. FIG. 1 is a schematic view for illustrating an apparatus which is to continuously machining ceramic valve preforms each having a rotation symmetrical shape. The ceramic valve preforms are machined according to the following four steps (1) to (4).
(1) The ceramic valve preform is conveyed to a station 1 from a feed side by means of a chuck C1-1, and chucked by a chuck C1-2. Then, a head portion and a rear end portion of a shaft of the valve preform shown by circles in FIG. 1 are ground by a grinding tool not shown. PA1 (2) The ground ceramic valve preform is chucked by the chuck C1-1, and placed on a reversing station where the preform is reversed at 180. PA1 (3) Then, the reversed ceramic valve is arranged between chuck heads C2-2a and C2-2b at a station 2 by means of a chuck C2-1, and is finish ground by a finish grinding tool not shown. PA1 (4) The finish ground ceramic valve is conveyed to a discharge side. PA1 (1) Said edge portion of the ceramic article is included in any one of an edge face, a peripheral face continuing to the edge face, an inclined face continuing to the peripheral face, and a concaved face continuing to the inclined face, and said consecutive machining steps and said machining directions are selected such that a side of a machined part of the ceramic article through which a grinding tool left in a certain machining step is set at a side of a part of the ceramic article through which the grinding tool enters in any succeeding step. PA1 (2) The process for machining the edge portion of ceramic article preform without chipping according to the present invention is particularly suitable for grinding an end portion of a head portion and a rear end portion of a shaft of a ceramic valve preform, end portions of ceramic tube, edge portions of a ceramic support pin jig, edge portions of an all-ceramic turborotor, etc., each having a rotation symmetrical shape. PA1 (3) Said machining of said lastly remaining part of the edge portions of the ceramic article preform in said final step without chipping is to machine an end face of the ceramic valve preform, said end face exteding in a direction substantially perpendicular to the axis in said rotation or being convexed in an axially outward direction.
The chipping phenomenon discussed in the present application occurs in the grinding step in the station 1.
FIG. 2 shows portions of the ceramic valve preform W chucked by the chuck C1-2 in the station 1, the portions being to be machined by a grinding stone T as a grinding tool in the state that the valve preform is being rotated around its axis. FIGS. 3 and 4 show the order and directions for machining a head portion and a rear end portion of a shaft of the valve preform according to a conventional machining process, respectively. In this conventional machining process, in order to decrease a grinding load, the movement of the grinding stone is controlled under the rotation of the valve preform W. As to the valve head portion, the grinding stone is moved from right to left as shown by a reference number "1" in FIG. 3 to grind a peripheral face of the head portion. Then, the grinding stone is moved obliquely upwardly from left to right as shown by a reference numeral "2". Thereafter, an end face of the valve head portion is ground by moving the grinding stone not through the entire end face but up to a central portion of the end face as shown by a reference numeral "3", different from the machining in the above "1" and "2". Finally, a corner portion of the head portion formed by the machining in the above "1" and "3" is chamfered by moving the grinding tool obliquely downwardly from left to right as shown in a reference numeral "4". Similarly, as to the rear end portion of the shaft of the valve preform, in order to decrease a grinding load, the movement of the grinding stone is controlled under the rotation of the valve preform W in FIG. 4. As to the rear end portion of the valve shaft, the grinding stone is moved from left to right as shown by a reference numeral "1" to grind a peripheral face of the shaft. Thereafter, as to an end face of the shaft, this end face is ground by moving the grinding stone not through the entire end face but up to a central portion of the end face as shown by a reference numeral "2", different from the machining in the above "1". Finally, a corner portion of the head portion formed by the machining in the above "1" and "2" is chamfered by moving the grinding tool obliquely downwardly from right to left as shown by a reference numeral "3". According to the above machining process, when the grinding stone grinds the ceramic valve preform through the entire grinding face, chipping may occur (See A and B in FIG. 3 and C in FIG. 4). Each of the valve head portion and the shaft portion of the ground ceramic valve preform is trimmed into a desired shape in the station 2 by the finish grinding (See shapes designated by dotted lines in FIGS. 3 and 4). However, there is possibility that a chipping trace remains even after the finish grinding. In order to completely remove the chipping trace, a finish grinding amount needs to be increased. FIGS. 5 and 6 are enlarged photographs for showing chipping occurred at an edge portion of the shaft and the head portion of the valve preform ground in the first station according to the conventional machining process, respectively (See B in FIG. 3 and C in FIG. 4). As is seen from these figures, the chipped portions remain in the head portion and the edge portion of the shaft of the valve.